The cytology and connection of cell classes in the medullary auditory nuclei will be studied by a variety of techniques. Following single unit recordings of physiological responses of cells in or around the lateral superior olive of the cat, horseradish peroxidase (HRP) will be iontophoresed extracellularly. The cells that are labelled by retrograde transport of the applied HRP will be identified by their locations and Nissl staining pattern. If necessary this will be accomplished by removal of the HRP reaction product. If it is not already known, the morphology of the cell classes that are so identified will be studied using Golgi, reduced silver, histochemical and immunohistochemical techniques. The same cytological studies will be done on cells within the lateral superior olive. Studies of the cytology and connections of the cell classes in the cat will provide the foundation for comparisons with human material in which similar cytological and immunohistochemical studies will be made. By establishing the connections of cell classes and characterizing them cytologically in the cat, the connections of cell classes that are likewise cytologically characterized in the human can be inferred. Understanding of the organization of the auditory brainstem in both species is therby advanced.